The mechanism causing corn yield loss based on the presence of weeds early in the growing season is still a mystery, says Rosenberg.

"Early weed competition can affect yield if the crop is under stress such as drought, extreme wet conditions and cold soils, and when high weed seedling populations are present, and/or the crop is slow to establish," he said. "However, there is also some research evidence suggesting that corn may be able to detect light reflected by the weeds that causes it to adjust its growth to make it more competitive with weeds at the cost of reducing yield."

For example, Rosenberg says corn detecting the presence of weeds may increase shoot growth at the expense of root growth resulting in increased size disparities among corn plants that also compete with each other for light. He points to several studies that have demonstrated that uniform corn sizes are important to optimize yield.

The most competitive weeds in corn will be about 3-4 in. tall when the corn reaches the V3-V4 growth stage. Iowa State University research indicated these weeds can reduce yields by about 3 bu./acre for every day they are left uncontrolled.

"For example, one foxtail plant per foot of row reduced corn yields by 7 bu./acre according to research done in Iowa," Rosenberg says. "Similarly, SDSU research has indicated these weeds can reduce corn yield by approximately 1%/day starting at about one week after corn and weed emergence."

If weeds are controlled early, Rosenberg says crops can compete with later emerging weeds.

"If the crop has a head start of several weeks, the shade it produces inhibits the growth of the later-emerging weeds. Early closure of the canopy is enhanced by narrow row spacing and adequate plant populations," he says.

In addition to preventing yield loss, he says early applications are becoming increasingly important just to get adequate weed control.


"Lambsquarters is a good example of a weed that is becoming increasing tolerant to glyphosate, particularly as it gets larger," he says.

Rosenberg points to a greenhouse study in which lambsquarters that was surviving field applications of glyphosate, SDSU was able to control these lambsquarters at normal rates when it was less than 4 in. tall but lambsquarters that was 8 in. tall tolerated Roundup WeatherMax applied at 66 oz./acre.

"It is interesting to note that some glyphosate product labels suggest controlling lambsquarters up to 12 in. tall," he says. “This may be possible yet under ideal conditions, but the most consistent weed control will be possible when controlling weeds less than 4 in. tall."

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